1) Field of the Invention
The present invention relates to a misfire detector that detects misfire of an internal combustion engine.
2) Description of the Related Art
Sometimes misfire occurs in internal combustion engines. The misfire may occur due to various reasons such as inadequate temperature in a combustion chamber, abnormality of an ignition system, and so on. When a misfire occurs, an engine speed drops so that engine torque that should originally be obtained is not output. Moreover, incompletely combusted gas that contains a large amount of hydrocarbon (HC) is discharged, which causes air pollution, moreover, a catalyst for purifying exhaust gas is damaged.
Japanese Patent Application Laid Open Publication (JP-A) No. H04-265475 discloses a misfire detector that detects a misfire. This misfire detector first obtains a value used to determine the combustion state of a cylinder in a current combustion stroke or of a cylinder in the immediately preceding combustion stroke (hereinafter, “cylinder in the preceding combustion stroke”). The value is obtained from the amount of motion when fuel is cut, and a misfire determination level in a misfire determination level map of the cylinder in the current combustion stroke or of the cylinder in the preceding combustion stroke is updated using the value.
Consequently, during normal operation, the misfire detector measures the amount of motion of the cylinder in the current combustion stroke or of the cylinder in the preceding combustion stroke in a predetermined zone upon fuel injection. If the amount of motion measured is equal to the misfire determination level or below, then the misfire detector determines that the amount of motion measured indicates misfire.
In the internal combustion engine, friction occurs between a piston and a cylinder or a journal and a bearing so that friction torque is generated depending on the amount of motion. The friction torque is generally correlated to temperature of lubricating oil between the piston and the cylinder and an engine speed. However, the correlation is not a linear correlation because it is a viscosity coefficient that changes caused by influence of the temperature of the lubricating oil. Therefore, generally, the friction torque is calculated using a map or a polynomial expression with a water temperature and the number of revolutions.
However, the characteristics of the friction torque change by degradation or the viscosity of the lubricating oil. The change is less correlated to the water temperature and the number of revolutions. Therefore, if such a change occurs, it is impossible to calculate accurate friction torque from the map or the like. Furthermore, how to determine the friction torque is not described in JP-A No. H04-265475. Therefore, if a difference occurs between the misfire determination level map updated and a misfire determination level map that should be obtained from a value corresponding to the actual amount of motion, and if the lubricating oil is degraded, the misfire cannot be detected with high precision.